1. Field of the Invention
This invention relates to the testing of underground formations or reservoirs. More particularly, this invention relates to a method and apparatus for isolating a downhole reservoir and testing the reservoir fluid.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. A large proportion of the current drilling activity involves directional drilling, i.e., drilling deviated and horizontal boreholes to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth""s formations. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at an end thereof that is rotated by a drill motor (mud motor) and/or by rotating the drill string. A number of downhole devices placed in close proximity to the drill bit measure certain downhole operating parameters associated with the drill string. Such devices typically include sensors for measuring downhole temperature and pressure, azimuth and inclination measuring devices and a resistivity-measuring device to determine the presence of hydrocarbons and water. Additional down-hole instruments, known as logging-while-drilling (LWD) tools, are frequently attached to the drill string to determine the formation geology and formation fluid conditions during the drilling operations.
Drilling fluid (commonly known as the xe2x80x9cmudxe2x80x9d or xe2x80x9cdrilling mudxe2x80x9d) is pumped into the drill pipe to rotate the drill motor, provide lubrication to various members of the drill string including the drill bit and to remove cuttings produced by the drill bit. The drill pipe is rotated by a prime mover, such as a motor, to facilitate directional drilling and to drill vertical boreholes. The drill bit is typically coupled to a bearing assembly having a drive shaft, which in turn rotates the drill bit attached thereto. Radial and axial bearings in the bearing assembly provide support to the radial and axial forces of the drill bit.
Boreholes are usually drilled along predetermined paths and the drilling of a typical borehole proceeds through various formations. The drilling operator typically controls the surface-controlled drilling parameters, such as the weight on bit, drilling fluid flow through the drill pipe, the drill string rotational speed and the density and viscosity of the drilling fluid to optimize the drilling operations. The downhole operating conditions continually change and the operator must react to such changes and adjust the surface-controlled parameters to optimize the drilling operations. For drilling a borehole in a virgin region, the operator typically has seismic survey plots which provide a macro picture of the subsurface formations and a pre-planned borehole path. For drilling multiple boreholes in the same formation, the operator also has information about the previously drilled boreholes in the same formation.
Typically, the information provided to the operator during drilling includes borehole pressure and temperature and drilling parameters, such as Weight-On-Bit (WOB), rotational speed of the drill bit and/or the drill string, and the drilling fluid flow rate. In some cases, the drilling operator also is provided selected information about the bottom hole assembly condition (parameters), such as torque, mud motor differential pressure, torque, bit bounce and whirl etc.
Downhole sensor data are typically processed downhole to some extent and telemetered uphole by sending a signal through the drill string, or by mud-pulse telemetry which is transmitting pressure pulses through the circulating drilling fluid. Although mud-pulse telemetry is more commonly used, such a system is capable of transmitting only a few (1-4) bits of information per second. Due to such a low transmission rate, the trend in the industry has been to attempt to process greater amounts of data downhole and transmit selected computed results or xe2x80x9canswersxe2x80x9d uphole for use by the driller for controlling the drilling operations.
Commercial development of hydrocarbon fields requires significant amounts of capital. Before field development begins, operators desire to have as much data as possible in order to evaluate the reservoir for commercial viability. Despite the advances in data acquisition during drilling using the MWD systems, it is often necessary to conduct further testing of the hydrocarbon reservoirs in order to obtain additional data. Therefore, after the well has been drilled, the hydrocarbon zones are often tested with other test equipment.
One type of post-drilling test involves producing fluid from the reservoir, shutting-in the well, collecting samples with a probe or dual packers, reducing pressure in a test volume and allowing the pressure to build-up to a static level. This sequence may be repeated several times at several different depths or point within a single reservoir and/or at several different reservoirs within a given borehole. One of the important aspects of the data collected during such a test is the pressure build-up information gathered after drawing the pressure down. From these data, information can be derived as to permeability, and size of the reservoir. Further, actual samples of the reservoir fluid must be obtained, and these samples must be tested to gather Pressure-Volume-Temperature and fluid properties such as density, viscosity and composition.
In order to perform these important tests, some systems require retrieval of the drill string from the borehole. Thereafter, a different tool, designed for the testing, is run into the borehole. A wireline is often used to lower the test tool into the borehole. The test tool sometimes utilizes packers for isolating the reservoir. Numerous communication devices have been designed which provide for manipulation of the test assembly, or alternatively, provide for data transmission from the test assembly. Some of those designs include mud-pulse telemetry to or from a downhole microprocessor located within, or associated with the test assembly. Alternatively, a wire line can be lowered from the surface, into a landing receptacle located within a test assembly, establishing electrical signal communication between the surface and the test assembly. Regardless of the type of test equipment currently used, and regardless of the type of communication system used, the amount of time and money required for retrieving the drill string and running a second test rig into the hole is significant. Further, if the hole is highly deviated, a wire line can not be used to perform the testing, because to test tool may not enter the hole deep enough to reach the desired formation.
A more recent system is desclosed in U.S. Pat. No. 5,803,186 to Berger et al. The ""186 patent provides a MWD system that includes use of pressure and resistivity sensors with the MWD system, to allow for real time data transmission of those measurements. The ""186 device allows obtaining static pressures, pressure build-ups, and pressure draws-downs with the work string, such as a drill string, in place. Also, computation of permeability and other reservoir parameters based on the pressure measurements can be accomplished without pulling the drill string.
The system described in the ""186 patent decreases the time required to take a test when compared to using a wireline. However, the ""186 patent does not provide an apparatus for improved efficiency when wireline applications are desirable. A pressure gradient test is one such test wherein multiple pressure tests are taken as a wireline conveys a test apparatus downward through a borehole. The purpose of the test is to determine fluid density in-situ and the interface or contact points between gas, oil and water when these fluids are present in a single reservoir.
Another apparatus method for measuring formation pressure and permeability is described in U.S. Pat. No. 5,233,866 issued to Robert Desbrandes, hereinafter the ""866 patent. FIG. 1 is a reproduction of a figure from the ""866 patent that shows a drawdown test method for determining formation pressure and permeability.
Referring to FIG. 1, the method includes reducing pressure in a flow line that is in fluid communication with a borehole wall. In Step 2, a piston is used to increase the flow line volume thereby decreasing the flow line pressure. The rate of pressure decrease is such that formation fluid entering the flow line combines with fluid leaving the flow line to create a substantially linear pressure decrease. A xe2x80x9cbest straight line fitxe2x80x9d is used to define a straight-line reference for a predetermined acceptable deviation determination. The acceptable deviation shown is 2xcfx80 from the straight line. Once the straight-line reference is determined, the volume increase is maintained at a steady rate. At a time t1, the pressure exceeds the 2xcfx80 limit and it is assumed that the flow line pressure being below the formation pressure causes the deviation. At t1, the drawdown is discontinued and the pressure is allowed to stabilize in Step 3. At t2, another drawdown cycle is started which may include using a new straight-line reference. The drawdown cycle is repeated until the flow line stabilizes at a pressure twice. Step 5 starts at t4 and shows a final drawdown cycle for determining permeability of the formation. Step 5 ends at t5 when the flow line pressure builds up to the borehole pressure Pm. With the flow line pressure equalized to the borehole pressure, the chance of sticking the tool is reduced. The tool can then be moved to a new test location or removed from the borehole.
A drawback of the ""866 patent is that the time required for testing is too long due to stabilization time during the xe2x80x9cmini-buildup cycles.xe2x80x9d In the case of a low permeability formation, the stabilization may take from tens of minutes to even days before stabilization occurs. One or more cycles following the first cycle only compound the time problem.
Still another known method for measuring permeability and other parameters of a formation and fluid is described in U.S. Pat. No. 5,708,204 issued to Ekrem Kasap, and assigned Western Atlas, hereinafter the ""204 patent and incorporated herein by reference. The ""204 patent describes a wireline method using a closed-loop system. One drawback to the ""204 patent is that it is only useful in wireline applications. A significant advantage of the present invention apparatus and method is the use of a MWD tool. Another improvement of over the ""204 patent is the use of varying draw-rates during a test as will be described in detail later.
Whether using wireline or MWD, the formation pressure and permeability measurement systems discussed above measure pressure by drawing down the pressure of a portion of the borehole to a point below the expected formation pressure in one step to a predetermined point well below the expected formation pressure or continuing the drawdown at an established rate until the formation fluid entering the tool stabilizes the tool pressure. Then the pressure is allowed to rise and stabilize by stopping the drawdown. The drawdown cycle may be repeated to ensure a valid formation pressure is being measured, and in some cases lost or corrupted data require retest. This is a time-consuming measurement process.
The present invention addresses some of the drawbacks described above by utilizing a closed-loop apparatus and method to perform formation pressure and permeability tests more quickly than the devices and methods described above. With quicker formation testing, more tests providing actual pressures and permeability may be provided to enhance well operation efficiency and safety.
The present invention provides an apparatus and method capable of creating a test volume within a borehole, and incrementally decreasing the pressure within the test volume at a variable rate to allow periodic measurements of pressure as the test volume pressure decreases. Adjustments to the rate of decrease are made before the pressure stabilizes thereby eliminating the need for multiple cycles. This incremental drawdown apparatus and method will significantly reduce overall measurement time, thereby increasing drilling efficiency and safety.
Further, an apparatus is provided for testing an underground formation parameter of interest such as a pressure during drilling operations. The apparatus has a drill string for drilling a well borehole. At least one extendable member is mounted on the drill string, and the at least one extendable member is selectively extendable into sealing engagement with the wall of the bore hole for isolating a portion of the annular space between the drill string and the borehole. A port in the drill string is exposable to formation fluid in the isolated annular space, and a fluid volume control device mounted within the drill string is in fluid communication with the port for incrementally reducing a pressure within the isolated portion. A sensor is operatively associated with the fluid volume control device for sensing at least one characteristic of the fluid.
In addition to the apparatus provided, a method is provided for testing an underground formation during drilling operations. The method comprises drilling a borehole with a drill string, isolating a portion of the annular space between the drill string and the borehole with at least one extendable member mounted on the drill string. The extendable member is brought into sealing engagement with the wall of the borehole, then a port disposed in the drill string is exposed to formation fluid in the isolated annular space. The method also included incrementally reducing pressure within the isolated portion of the annulus with a fluid volume control device mounted within the drill string, and sensing at least one characteristic of the fluid with a sensor.
A wireline apparatus is provided for determining an underground formation parameter of interest such as contact points. The apparatus has a tool disposed on a wireline used to lower the tool into a well borehole. At least one extendable member is mounted on the tool, and the at least one extendable member is selectively extendable into sealing engagement with the wall of the borehole for isolating a portion of the annular space between the tool and the borehole. A port in the tool is exposable to formation fluid in the isolated annular space, and a fluid volume control device mounted within the tool is in fluid communication with the port for incrementally reducing a pressure within the isolated portion. A sensor is operatively associated with the fluid volume control device for sensing at least one characteristic of the fluid.